1. Field of the Invention
The present invention relates to an improved frame structure for fixing a shadow mask in a flat color CRT(Cathode Ray Tube).
2. Description of the Related Art
In general, the flat color CRT is an apparatus for scanning electron beams to phosphor dots applied on a screen according to image information.
As shown in FIG. 1, the electron beams are color selected while passing through a shadow mask 10, which is formed of a thin film with a number of slots 11 and fixed at the rear of the screen by a frame structure.
The frame structure comprises main frames 20a directly welded to the shadow mask 10, and sub-frames 30 for supporting the lower parts of the main frames 20a while tensioning the same.
To be specific, the upper part of each of the main frames 20a defines a section perpendicular to the shadow mask 10 and the lower part is bent inwardly about the shadow mask in a right angle with the end reaching the perpendicular section so that the cross sectional configuration in the x-axis direction defines a triangle.
The sub-frames 30 are shaped as a bar. The shadow mask is tensioned and fixed to the frame structure according to the following process.
After the sub-frames 30 are welded to both end lower parts of the main frames 20a, when the main frames are forced so that the shadow mask 10 having a small amount of volume can be placed on the main frames, the sub-frames 30 are bent.
After the shadow mask 10 is welded to the upper parts of the main frames 20, when the force is released from the main frames 20a, the sub-frames 30 return to the original position while tensioning the shadow mask 10.
However, since the upper parts of the main frame perpendicular sections are very narrow, it is not easy to weld the shadow mask to the upper parts of the main frames and thus productivity is lowered.
Also, the main frames have a small amount of stiffness, and are easily deformed in tensioning the shadow mask so that the shadow mask may not be tensioned with a uniform tensile force.
Therefore, a resistant force generated to the shadow mask corresponding to the tensile force is distributed different according to the position so that the shadow mask has a low vibration frequency in a middle portion and a high vibration frequency at both ends thereby having a wide range of amplitude.
In other words, in use of the CRT, vibration from a speaker is amplified in a cabinet for defining a front outer case of the CRT and amplified vibration is transferred to a panel side of the CRT, then to the main frames and the shadow mask through springs in the frame.
When vibration transferred to the shadow mask is identical with the vibration frequency of the shadow mask, resonance takes place and slots tremble to distort the incident direction of the electron beams so that howling takes place to create stripes on the screen and definition is degraded.
It is therefore an object of the invention to provide a flat color CRT in which a frame structure for supporting a shadow mask in a shadow mask assembly is modified to simplify a welding operation and howling of the shadow mask is reduced to reproduce clean definition.
According to an embodiment of the invention to obtain the object, it is provided a shadow mask assembly comprising: a shadow mask having an effective area with slots and a non-effective area without slots in the periphery; and a frame fixing body for tensioning the shadow mask; wherein the shortest distance tM from the outermost end of a main frame welded to the shadow mask to the interface between the effective area and the non-effective area and the width tW of the mask welding part formed at the upper part of the main frame have the relation of   0.14  ≤            t      W              t      M        ≤      1.0    .  
It is preferred that the shadow mask is welded to the outermost end of the main frame in the effective area side direction in the range of tW/2 to tW.
It is more preferred that the main frame and the shadow mask are weld in the range of   0.30  ≤            t      M              t      W        ≤      0.99    .  